Synaptic mechanisms in experience-dependent processes responsible for the development of central neural maps of visual space

Experience-dependent synaptic plasticity has been demonstrated in the binocular visual system of the aquatic frog Xenopus laevis. This thesis offers further support for the role of correlated patterns of neural activity in this synaptic plasticity and demonstrates, for the first time, that N-methyl-D-aspartate (NMDA)-type glutamate receptors contribute to synaptic transmission in the optic tectum of Xenopus. Electrophysiological mapping techniques assessed the capacity for synaptic reorganisations to occur in the binocular visual system of Xenopus. The intertectal system, a series of connections that mediates binocular vision in lower vertebrates, reorganised its pattern of synaptic connections in response to large changes in the orientation of both eyes. Stroboscopic illumination was shown to interfere with correlated patterns of neural activity in the tectum, but the development of the retinotectal projection was not affected by this procedure. However, the intertectal systems capacity to modify its pattern of synaptic connections, in response to changes in eye position, was disturbed by this visual environment. Next, in vivo and in vitro preparations of the optic tectum were developed to monitor pre and postsynaptic activity in the visual system of Xenopus. It was established that mono-synaptic transmission between retinal fibres and tectal neurons was mediated by non-NMDA type glutamate receptors. However, when the optic tract of the adult animal was electrically stimulated the late poly-synaptic (U2) component of the evoked response was reduced by ˜30% in the presence of 50μM AP5. This NMDA mediated component of the late U2 response was at its most significant 1 month after metamorphosis, was regulated by visual experience, and was not present in another species of frog, Rana pipiens. Finally, a thin slice preparation of the optic tectum was developed which enabled tight-seal whole cell recordings to be made from identified tectal neurons